My opinion is broadly generalized but considering iOS demographics they would be more easily affected by a social engineering attempt to gain access to the device to install the RAT although I don’t have any evidence to support this that’s not anecdotal. If you can get the phone unlocked you can sideload whatever you want on it. Doing this remotely would be the main hurdle. You would need physical access to the device at least once to set it up. Or get the user to set it up unknowingly. With the amount of steps involved I doubt it would be something someone will just accidentally click on or that you could install without knowing the credentials to the device. Apple has multiple steps in place to prevent accidental installation of apps and even then you’d need to know how to cover your tracks or it would be easily found on the device. Even routine features like battery and network usage could potentially disclose the presence of a RAT on the device. Apple does a great job at product security partly due to the fact that they will pay large sums of money for verified exploits. If you happen to find one get in contact with their security team. You might just be the next big payout.

TL:DR - Your teachers right it’s nearly impossible. I would also add easily found, and easily removed.